In recent years, as semiconductor elements become more highly integrated, metallic interconnect lines of Al, Cu, and the like have become narrower in width. This has given rise to a demand for improvement in an electromigration property. In order to improve the electromigration property, it is desirable that in crystal growth of wiring metal after the deposition, the densest surface of crystals of the metal grows in a pillar shape, and for this, it is desirable that the sputtering is performed so that a densest surface of a barrier film of TiN, TaN, or the like formed under the wiring metal also becomes in a pillar shape.
When a Cu interconnect line is used, its barrier film is most suitably a TaN (tantalum nitride) film. The TaN film is obtained through deposition using a high-purity Ta sputtering target in a nitride atmosphere. To perform sputtering so that the densest surface of the barrier film becomes in the pillar shape requires miniaturizing a crystal grain size of target metal and making crystal orientation random. Further, in order to improve uniformity of a sputtered film, residues of a cast structure (ghost grains) have to be eliminated.
In the manufacture of a Ta sputtering target, it has been conventionally known that an ingot is subjected to cold upset forging after subjected to cold side forging. The side forging is a process to apply a pressure to a columnar ingot in its diameter direction. Further, the upset forging is a process in which the ingot extended in its longitudinal direction by the side forging is worked in the longitudinal direction into a thin plate. The both are processes to cause plastic deformation by applying a pressure in a fixed direction. However, when an average crystal grain size becomes microscopic, the plastic deformation only by the pressure in the fixed direction is likely to cause crystal orientation to be in a specific direction.
In the manufacture of the Ta sputtering target, cold knead forging has been known. The knead forging is a working method of plastically deforming an ingot by alternately applying pressures in its thickness direction and diameter direction. The successive application of the pressures in the thickness direction and the diameter direction decreases the likelihood of the occurrence of specific crystal orientation. When a Ta sputtering target having a microcrystalline structure with a 50 μm average crystal grain size or less is manufactured, uniform random orientation cannot be necessarily obtained in the thickness direction even if uniform random orientation is obtained on a sputtered surface. This has a problem that a uniform film cannot be obtained in the long-time sputtering due to a disadvantage such as variation in a sputtering rate.